Reheater steam generating unit with gas recirculation for reheat control



P. PACAULT Filed Jan. 17, 1956 REHEATER STEAM GENERATING UNIT WITH GASRECIRCULATION FOR REHEAT CONTROL 30 4 27 FIG] 31 7 25 Q 16 l f- '-.f 19

SPHTR 1 m Xi 'INVIENTOR. Pierre Pacaulr ATTORNEY United States PatentREHEATER STEAM GENERATING UNIT WITH GASLRECIRCULATION FOR REHEAT CON-,TRO

Pierre Pacault, Paris, France, assignor to The Babcock & Wilcox Company,New York, N.Y., a corporation of New Jersey This invention relates to amethod and apparatus for generating high pressure vapor, superheatingthe generated vapor, reheating the generated vapor, and controlling thetemperature of vapor superheating and vapor reheating over a wide rangeor rate of vapor generation. The type of unit with which the inventionis concerned is particularly adapted for supplying high temperaturevapor or steam to prime movers such as plural stage steam turbines, incentral power stations.

In steam generators of the pertinent type, including convectionsuperheating and reheating units, both the superheat temperature and thereheat temperature drop from an optimum value as the load is reducedfrom *control'point load,- it no provisions are made for correcting thisinherent action. The reheat temperature drops at a greater rate than thehigh pressure superheat temperature inasmuch as the superheating takesplace from a substantially constant saturated temperature of the highpressure steam, while reheating is from the tempcrature of the lowpressure steam, this temperature decreasing as the steam flow throughthe turbine is reduced.

For the highest thermo-dynamic efliciency of such a steam turbine, it isnot only important that the superheated steam be supplied to the turbineat a substantially uniform temperature but it is also important tosupply the reheated low pressure steam at a uniform temperature over awide load range.

'Because of the above indicated situation, and in order to provideuniformity of steam temperatures of both the high pressure superheatedsteam and the low pressure reheated steam, there must be differentcontrol provisions or means having different degrees of effectiveness onthe superheated steam and on the reheated steam. It is the aim of thepresent invention to provide apparatus and a method of control whichwill effect substantially uniform superheat and uniform reheat steamdelivery over a wide range of generator output.

I More specifically, the invention is concerned with vapor generatingand vapor heating units of the type using low temperature gasrecirculation for the control of the heated vapor temperature. When theunit involves a furnace lined with vapor generating tubes constituting aradiant section, and when the furnace is supplied with high temperatureheating gases by the burning of a slag forming fuel, the gasrecirculation of the invention is '80 applied and controlled as tominimize slagging of the vapor heating surfaces disposed within heatinggas passages leading from the furnace. With this arrangement, the mostexpensive parts of high capacity units of the pertinent type, i.e.,radiant section of the furnace with its wall tubes can be substantiallyreduced in size and cost.

A specific embodiment of the invention also involves the recirculationof gases for the regulation of the final 'vapor temperature. Byrecirculating gases to the furnace or the radiant vapor generatingsection of the unit, the gases having been cooled by heat transmissionthere- Patented Sept. 22, 1959 from to the convection vapor heatingsurfaces, the radiant heat absorption may be reduced at low loads sothat more heat is available for the convection section for vaporsuperheating and/ or vapor reheating. With a unit having different gasflow paths containing, respectively, convection vapor superheatingsurface and convection vapor reheating surface, the total gas flow maybe proportioned between the gas flow paths by damper means in order toregulate the final temperature of vapor superheat or vapor reheat, theother of these temperatures being regulated by control of the rate ofgas recirculation.

By way of example, vapor generating and vapor heating units having a gasrecirculation system or systems, and methods of operating vaporgenerating units involving the pertinent gas recirculation for thepulpose of effecting control of vapor temperature such as reheattemperature, are disclosed in the accompanying drawings and thefollowing description:

In the drawings:

Fig. 1 is a diagrammatic view illustrating a vapor geni erating,superheating, and reheating unit in which gas recirculation is involvedin a closed circuit for controlling the reheated vapor temperature;

Fig. 2 is a diagrammatic view in the nature of a vertical section online 2-2 of Fig. 3 illustrating a second and third modification; and

Fig. 3 is a horizontal section in the nature of a plan on the line 3-3of Fig. 2 and looking in the direction of the associated arrows, Fig. 3and Fig. 2 collectively, showing a third modification of the inventionwherein there are two gas recirculation systems one of which controlsreheat while the other controls superheat.

In each of the modifications shown in the drawings, there is a radiantsection including a vertically elongated furnace chamber 1. When theunit includes a natural circulation system for vaporizable liquid, thewalls and other boundary surfaces of the furnace chamber include uprightvapor generating tubes such as 2 and 3, incorporated into the fluidcirculation of the system by a steam and water drum 4, a lower header 5,and other appropriate circulatory connections.

The furnace chamber 1 is fired in its lower region by a variable ratefuel burning means 6 which may include pulverized coal burners, orcombination, pulverized coal, gas and oil burners. The unit of eachembodiment of the invention is so constructed as to provide a gas flowpath leading from the gas outlet of the furnace chamber to atmosphere.This gas flow path includes a lateral gas pass 7 leading from the upperportion of the furnace chamber and arranged above an arch 8 includingparts of the vapor generating tubes and projecting inwardly from thevertical plane of the rear furnace wall along the vapor generating tubes3. This gas flow path further includes a downfiow gas pass 9 conductingthe gases over a primary superheater and an economizer section 10 andleading thence over the surfaces of an air heater 11, and then through astack to the atmosphere.

Extending from the upper part of the furnace chamher is a reheater gaspass 12 which may be considered as a supplementary gas passage forming apart of a closed gas circuit of a gas recirculation system and havingdisposed therein a convection vapor reheater 13. At the entrance of thegas passage 12, the associated wall tubes 2 of the furnace chamber aremore widely spaced to form a screen 14 to permit the free entrance ofgases to the reheater gas pass 12 and to provide radiant heat protectionfor the reheater.

In addition to the lateral reheater gas passage 12, the closed gascircuit of the Fig. 1 unit includes a gas flow path partially withinfurnace chamber 1, a recirculated gas fan 15, fan inlet duct-work 16constituting the only gas outlet of the reheater gas pass 12 and leadingtherefrom to the inlet of the recirculated gas fan 15, and fan outletduct-work 17 leading from the outlet of the fan to the lower part of thefurnace chamber 1. The fan 15 is adapted to effect recirculation offurnace gases at a regulable rate through the reheater gas pass 12, theduct 16 and the duct-work 17 to the lower part of the furnace chamber.The duct work 17 terminates in suitable discharge means adapted to givethe most favorable orientation and distribution of recirculated gases inrelation to the combustion zone within the furnace chamber 1 Preferablythis duct work has a plurality of regularly spaced openings provided bythe screen parts 18 of the wall tubes 2 distributed throughout the widthof the associated furnace chamber wall.

Steam, or generated vapor, in the operation of the unit, flows throughsuitable circulators from the steam and water drum -4 to the inletheader 19 of the convection primary superheater 20. From the outletheader 21 of this superheater the superheated steam flows throughappropriate conduit means 22 and 23 and an interposed spray attemperator24 to the inlet header 25 of the secondary superheater 26. From theoutlet header 27 of this superheater, the superheated steam flows to thesteam inlet to the high pressure stage 28 of the steam turbine. From theexhaust of this turbine stage the lower pressure steam flows throughconduit means 29 to the reheater inlet header 30 and thence through theconvection reheater 13. From the outlet header 31 of the reheater thesteam flows through conduit means 32 to the steam inlet of the lowerpressure turbine stage 33.

The walls of the superheater gas pass 7 and the reheater gas pass 12 areof appropriate construction and they may include vapor generating tubesconnected into the fluid circulation of the furnace chamber in the samemanner as that in which the furnace chamber wall tubes are connected.

In addition to the reheater 13, the closed gas path for the reheater mayalso contain a superheater section, and, in some cases, this gas flowpath may be also provided with heat exchange surfaces other than vaporheating surfaces, such as the surfaces of an economizer section.

In operation, a portion of the furnace gases, instead of flowingdirectly from the furnace chamber to the atmosphere through the gaspassages 7 and 9, is recirculated through the closed reheater circuitincluding the components 12, 15, 16 and 17 and thus in heat exchangerelationship with the heating surfaces of the reheater 13, and bycontrolling the rate of gas recirculation the final temperature of vaporreheat may be maintained at a predetermined value, over a wide range ofload or rate of vapor generation, thus overcoming the inherent tendencyof the convection reheater 13 to effect undesirably low reheattemperatures at low loads, and particularly those loads substantiallybelow normal load or control point load.

The degree to which the recirculated gases are cooled depends upon thetotal provisions within that passage for effecting heat transfer fromthe gases. The gases cooled by passing through the reheater gas passage12 and entering the furnace chamber at its lower part are subsequentlymixed with the unrecirculated products of combustion from the fuelburning means 6, before they reach the entrances of the gas passages 7and 12. The recirculated gases therefore have a tempering action whichmay be controlled, in the case of pulverized coal firing, to minimizeslag accumulations upon the convection surfaces exemplified by thereheater 13 and the secondary superheater 26. The recirculated gasesalso reduce the residence time of the unrecirculated gases Within thefurnace, and they reduce the area and volumetric extent of the heatemitting unrccirculated gases and thereby reduce heat radiation to thevapor generating wall tube surfaces of the furnace chamber, thusproviding a higher proportion of heat in the gases available forreheating the generated vapor.

The illustrative unit also provides for the highest permissible gastemperature at the entry of the reheater gas pass 12, thus permittingthe attainment of a predetermined high reheat temperature at low loads,and minimizing extent and cost of the reheater. Moreover, the attainmentof optimum superheat temperature is facilitated since none of the heatin the gases flowing through the gas passes 7 and 9 is required forreheating. Economy in the extent and cost of the superheater istherefore attained. In addition, cooling of the furnace gases to therequisite degree, before reaching the convection heat exchange surfacesprovides for the reduction in size and cost of the radiant section ofthe unit.

Placing the reheater within the supplementary gas passage 12 rather thanwithin the gas flow path leading to atmosphere provides for betterprotection of the reheater during periods of disturbance such as duringoutages of the generator driven by the steam turbine, and duringstarting up of the unit. The arrangement facilitates protectiveregulation of superheat and reheat temperatures during starting up ofthe unit.

Fig. 2 of the drawings may be considered as illustrating twomodifications of the Fig. 1 unit. Each of these additional modificationsinvolves the primary superheater 40, the intermediate superheater 42 andthe final or secondary superheater 44, with the attemperator '46disposed in the line 48 connecting the outlet header 50 of theintermediate superheater 42 to the inlet header 52 of the finalsuperheater 44. It will appear from the drawing that the secondmodification is otherwise similar to the Fig. 1 unit.

The third modification is considered as illustrated by Figs. 2 and 3when considered together. In this third modification, the secondarysuperheater 44 is disposed in a gas passage 56 which is a part of one oftwo separate gas recirculation systems. The first of these systemsincludes the recirculated gas fan 58, the fan outlet duct work 60, andthe fan inlet duct work 62 which is the only gas outlet from the gasexit of the gas passage 56. In the third modification, there is a secondand separate gas recirculation system located in parallelism withrespect to the gas recirculation system including the gas passage 56 andthe secondary superheater 44. This second parallel gas passage isindicated in Fig. 3 as having therein a reheater 64 which is similar tothe reheater 13 of the Fig. 1 modification. Otherwise, this second gasrecirculation system is similar to that involved in either one of theFig. 1 and Fig. 2 modifications.

In the operation of the illustrative unit of Fig. 1, the flow ofrecirculated gases into the furnace chamber 1 may be controlled from aplurality of operative influences or variables. For example, such flowof recirculated gases may be controlled from representations of gas temperature at the position 70 immediately in front of the secondarysuperheater 26. Another variable in the control of such recirculated gaswould be representations of the outlet temperature of the reheat vaporor steam at such a position as that indicated at 72, in the lineconducting reheated vapor .to the turbine stage 33.

Such control may be manual or it may be automatic. In the latter case,it would be effected by known control systems such as that indicated inthe co-pending application 193,832 filed in November 3, 1950 nowabandoned and belonging to the same assignee.

Additional control of the Fig. 1 unit would preferably involve thecontrol of the rate of firing of the fuel burning means 6 fromrepresentations of steam or vapor flow at such a position as thatindicated at 74, in the line 76 leading from the secondary superheateroutlet header 27 to the steam inlet of the high pressure turbine stage28. Another simultaneously effective influence upon the rate of fuelfiring would be the pressure of g. the vapor at such a position as thatindicated Further control of the operation of the Fig. 1 unit might alsoinvolve controls such as those disclosed in the pending applications, toMoonan Ser. No. 542,927, Troutman Ser. No. 542,925, and Stallkamp Ser.No. 542,926 (filed -2655), for so controlling the operation of therecycled gas system and more particularly, the gas flow in the vicinityof the recycled gas fan, that any reverse flow of gases from the furnaceinto the normal outlet of the recycled gas fan is prevented at low ratesof flow of recycled gas.

The second modification illustrated by Fig. 2 alone would have controlssimilar to those of the Fig. 1 unit but, of course, the vaportemperature control would be in the line leading from the outlet of thesecondary superheater 44.

The control of the third modification illustrated by Figs. 2 and 3, whenconsidered together, would be similar to the above indicated controls,modified by the inclusion of superheat temperature control for thesuperheater recycled gas system including the gas pass '56, andincluding reheater outlet vapor temperature control for the recycle gasflow in the parallel recycled gas system including the reheater -64.

As further illustrative of the steam temperature control of theinvention, the attemperator, such as indicated at 24 or 46, may be ofthe type indicated in the US. patent to Fletcher et al., 2,550,683, ofMay 1, 1951.

Although the invention has been described with reference to a fewspecific modifications it is to be recognized that the invention is notlimited to all of the details of any one of these modifications. It israther to be considered as of a scope commensurate with the scope of thesubjoined claims.

What is claimed is:

1. In a fluid heat exchange unit, a single furnace having vaporgenerating wall tubes, fuel burning means for supplying the furnace withhigh temperature heating gases for the radiant transmission of heattherefrom to the fluid within the tubes, means forming a gas flow pathleading from the furnace gas outlet to the atmosphere, vapor superheatersubject to the heat of the gases in said gas flow path and beyond thegas outlet of the furnace, means forming with a part of the furnace aclosed gas flow path leading from another part of the furnace adjacentits gas outlet and upstream of said vapor superheater to a positionwithin the furnace remote from said gas outlet, other fluid heatexchange means disposed within said closed gas flow path and subject tothe heat of the gases therein, and means for controlling the flow ofrecycled gases through said closed gas flow path to thereby control thetemperature of the fluid heated by said other fluid heat exchange means.

2. In a fluid heat exchange unit, a single furnace having vaporgenerating wall tubes, fuel burning means for supplying the furnace withhigh temperature heating gases for the radiant transmission of heattherefrom to the fluid Within the tubes, means forming a gas flow pathleading from the furnace gas outlet to the atmosphere, a vaporsuperheater subject to the heat of the gases in said gas flow path andextending beyond the gas outlet of the furnace, means forming with apart of the furnace a closed gas flow path leading from another part ofthe furnace adjacent its gas outlet and upstream of said vaporsuperheater to a position within the furnace remote from said gas outletand spaced from the position of fuel burning, other vapor heating meansdisposed within said closed gas flow path and subject to the heat of thegases therein, and means for controlling the flow of recycled gasesthrough said closed gas flow path to thereby control the temperature ofthe fluid heated by said other vapor heating means.

3. In a fluid heat exchange unit, a single furnace having vaporgenerating wall tubes, fuel burning means for at 74 in 6 supplying thefurnace with high temperature heating gases for the radiant transmissionof heat therefrom to the fluid within the tubes, means forming a gasflow path leading from the furnace gas outlet to the atmosphere,

11a. vapor superheater subject to the heat of the gases in said gas flowpath and extending beyond the gas outlet of the furnace, means formingwith a part of the furnace a closed gas flow path leading from anotherpart of the furnace adjacent its gas outlet and upstream of said vaporsuperheater to a position within the furnace remote from said gas outletand spaced from the position of fuel burning, a convection vaporreheater disposed within said closed gas flow path and subject to theheat of the gases therein, and means for controlling the flow ofrecycled gases through said closed gas flow path to thereby control thetemperature of the vapor exiting from the reheater.

4. In a vapor generating, vapor superheating, and reheating unit; meansforming a radiantly heated section including a single furnace chamberwith vapor generat ing wall tubes; gas passage means providing a gasflow path leading from the radiant section to atmosphere; fuel burningmeans providing the furnace chamber with high temperature heating gases;said gas passage means providing a superheater gas passage leading fromthe gas outlet of the furnce chamber; a convection superheater disposedwithin said gas passage; means providing a reheater gas passage separatefrom the superheater gas passage and leading from the furnace chamber ata position adjacent its gas exit and upstream of said superheater; a gasrecirculation system affording a closed gas circuit and including arecirculated gas fan and fan inlet duct work forming the gas outlet ofthe reheater gas passage, and fan outlet duct work communicating withthe furnace chamber at a location spaced from its fuel burning means;and a vapor reheater disposed within the reheater gas passage.

5. A steam boiler having walls defining a single furnace chamber, meansfor burning fuel in said furnace chamber, steam generating wall tubesarranged to absorb radiant heat from said furnace chamber, means forminga gas outlet flue opening to and arranged to receive heating gases fromsaid furnace chamber at a location remote from said fuel burning means,a convection heated steam superheater in said gas outlet flue, meansforming a second gas outlet flue opening to said furnace chamber at alocation spaced from said first gas outlet flue and remote from saidfuel burning means, a steam reheater in said second gas outlet flue, anda gas recirculating fan arranged to withdraw heating gases from saidfurnace chamber through said second gas outlet flue and to return saidheating gases to said furnace chamber at a location in the gas flow paththrough said furnace chamber upstream of said second gas outlet flue.

6. A steam boiler having walls defining a single furnace chamber, meansfor burning fuel in said furnace chamber, steam generating wall tubesarranged to absorb radiant heat from said furnace chamber, means forminga gas outlet flue opening to and arranged to receive heating gases fromsaid furnace chamber at a location remote from said fuel burning means,a convection heated steam superheater in said gas outlet flue, meansforming a second gas outlet flue opening to said furnace chamber at alocation spaced from said first gas outlet flue and remote from saidfuel burning means, a steam reheater in said second gas outlet flue, anda gas recirculating fan arranged to withdraw heating gases from saidfurnace chamber through said second gas outlet flue and to returnsubstantially all of the heating gases so withdrawn to said furnacechamber at a location in the gas flow path through said furnace chamberupstream of said first and second gas outlet flues.

(Other references on following page) References Cited in the file ofthis patent UNITED STATES PATENTS Lacerenza Mar. 25, 1952 KuppenheimerJuly 8, 1952 Blaskowski Aug. 3, 1954 Armacost et a1 1- Feb. 19, 1957 8FOREIGN PATENTS France Aug. 4, 1954 Great Britain July 24, 1940

